Hydroxy compounds (A2): phenol
A-Level Chemistry Topic 32 8:38 English narration · English + 中文 subtitles burned in
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In the eighteen sixties, surgery was terrifying.
在十九世纪六十年代，做手术非常可怕。
The cutting could be survived; the infection that followed often could not.
刀口本身还挺得过去，随后而来的感染却常常致命。
After an amputation, almost half of the patients died.
截肢之后，将近一半的病人死去。
Then a surgeon called Joseph Lister began washing wounds with a sharp-smelling liquid, carbolic acid, and the deaths fell sharply.
后来，一位名叫约瑟夫·李斯特的外科医生， 开始用一种气味刺鼻的液体——石炭酸——清洗伤口，死亡率随之大幅下降。
That liquid is today's molecule: phenol.
这种液体就是我们今天的分子：苯酚。
It carries a hydroxyl group, like an alcohol — but the group sits straight on a benzene ring, and that changes everything.
它像醇一样带着一个羟基， 但这个羟基直接连在苯环上，而正是这一点，改变了它的全部化学性质。
Two jobs today.
今天有两件事。
First, one last reaction of the alcohols: with an acyl chloride, to make an ester quickly and completely.
第一，醇的最后一个反应：与酰氯反应，快速而彻底地生成酯。
Then phenol itself — why it is acidic, why its ring reacts so easily, and a real exam question on both.
然后是苯酚本身——它为什么显酸性，它的环为什么这么容易反应， 以及一道同时考这两点的真题。
Let's begin.
让我们开始吧。
Start with the alcohols.
先说醇。
At A Level you meet one more way to make an ester: use an acyl chloride.
到了 A Level，你会学到制备酯的又一种方法：使用酰氯。
Shake ethanol with ethanoyl chloride at room temperature, with no heat and no catalyst, and you get ethyl ethanoate and hydrogen chloride.
把乙醇与乙酰氯在室温下摇匀，不加热、不用催化剂，就得到乙酸乙酯和氯化氢。
The hydrogen chloride escapes as misty white fumes — that is your observation.
氯化氢以白色雾状的烟逸出，这就是你在实验室看到的现象。
Inside the flask, the alcohol oxygen attacks the carbonyl carbon, and chloride leaves.
在瓶内，醇的氧原子进攻羰基碳，随后氯离子离去。
Best of all, the reaction goes to completion, so there is no equilibrium to fight.
最妙的是，这个反应能进行到底，不必再跟平衡较劲。
Why is that better?
为什么它更好？
Compare the route you learned last year: a carboxylic acid, concentrated sulfuric acid, heat, and a reversible reaction that never finishes.
和你去年学过的路线比一比：用羧酸，浓硫酸作催化剂，还要加热， 而且是一个永远做不完的可逆反应。
The acyl chloride needs none of it, because the chlorine pulls electrons away, leaving the carbonyl carbon strongly positive and chloride ready to go.
酰氯完全不需要这些， 因为氯把电子拉走，使羰基碳带很强的正电，而氯离子又随时准备离去。
Phenols react this way too, and the acid route works badly on a phenol.
酚也能这样反应，而羧酸路线对酚的效果很差。
So what counts as a phenol?
那么，什么才算酚？
In a phenol the hydroxyl group is joined straight onto a ring carbon.
在酚里，羟基直接连在环上的碳原子上。
Move it one carbon out, onto a side chain, and it is not a phenol any more: phenylmethanol is an ordinary primary alcohol.
把它往外挪一个碳，接到侧链上，它就不再是酚了：苯甲醇只是一个普通的伯醇。
Learn this pair, because the exam loves it.
要记住这一对，因为考试很爱考。
Four-methylphenol and phenylmethanol have the same molecular formula, yet they sit at opposite ends of the acidity list.
4-甲基苯酚和苯甲醇的分子式完全相同， 却分别站在酸性排序的两端。
Where does phenol come from?
苯酚是怎么制得的？
Start from phenylamine, a benzene ring carrying an amine group.
从苯胺出发——一个带着氨基的苯环。
Cool it in dilute acid, add sodium nitrite, and hold the mixture below ten degrees Celsius.
把它在稀酸中冷却，加入亚硝酸钠，并把混合物保持在十摄氏度以下。
That gives a diazonium salt, and the temperature is where the mark is, because a warm diazonium salt falls apart.
这样就得到重氮盐；温度正是得分点，因为重氮盐一旦变温就会分解。
Then warm that salt with water, and out comes phenol, with nitrogen gas bubbling off.
然后把这种盐与水加热，苯酚就出来了，同时有氮气冒出。
Now the big idea of the topic.
现在讲这个专题的核心思想。
Phenol can lose the hydrogen from its hydroxyl group, leaving the phenoxide ion.
苯酚可以失去羟基上的氢，留下苯氧负离子。
What makes phenol special is what happens next: a lone pair on the oxygen overlaps the ring, so the negative charge does not stay on the oxygen.
苯酚与众不同之处在于接下来发生的事：氧上的一对孤对电子与环重叠， 于是那个负电荷不再停留在氧上。
It is delocalised into the ring, shared over the whole system.
它离域进入环中，由整个体系共同分担。
A spread-out charge is a stable charge, so the anion is stabilised, and the hydrogen leaves more easily.
电荷分散开来就更稳定，所以这个负离子被稳定了，氢也就更容易失去。
The syllabus asks you to rank three: ethanol, water and phenol.
考纲要求你给三种物质排序：乙醇、水和苯酚。
Never compare the acids themselves — look at the anion left behind by each one.
千万不要直接比较这些酸本身——要看每一种留下的那个负离子。
Ethanol leaves an ethoxide ion, and its alkyl group pushes electron density onto an oxygen that already carries charge, so ethanol is the weakest.
乙醇留下乙氧负离子，而它的烷基把电子密度推向本来就带负电的氧，所以乙醇最弱。
Water leaves a hydroxide ion: nothing pushing, nothing spreading.
水留下氢氧根离子，既没有推电子，也没有电荷分散。
Phenol leaves a phenoxide ion that shares its charge with the ring, so phenol is the strongest of the three.
苯酚留下苯氧负离子，负电荷与环共同分担，所以三者之中苯酚最强。
Learn that order — the exam asks for it directly.
把这个顺序记牢——考试会直接考它。
Being acidic gives phenol two reactions you must know.
显酸性给了苯酚两个你必须掌握的反应。
With sodium hydroxide solution, phenol is neutralised to sodium phenoxide and water — and an alcohol does nothing at all, so this reaction alone tells a phenol from an alcohol.
与氢氧化钠溶液反应， 苯酚被中和成苯氧钠和水，而醇完全没有反应，所以仅凭这一个反应就能区分酚和醇。
With sodium metal, phenol gives sodium phenoxide and hydrogen gas; that equation was worth a mark in a recent paper.
与金属钠反应，苯酚生成苯氧钠和氢气。 这个方程式在最近的一份试卷中就值一分。
But alcohols react with sodium too, so sodium cannot separate them.
但醇也能与钠反应，所以钠无法把两者区分开。
Here is the identification question examiners love: three liquids and two reagents.
下面是考官最爱出的鉴别题：三种液体，两种试剂。
Ethanol, phenol and ethanoic acid, tested with sodium hydroxide and then with sodium carbonate.
乙醇、苯酚和乙酸，先用氢氧化钠试，再用碳酸钠试。
Sodium hydroxide dissolves phenol and the acid, and leaves ethanol alone.
氢氧化钠能溶解苯酚和乙酸，而乙醇没有反应。
Now add sodium carbonate: neither phenol nor ethanol is acidic enough to attack a carbonate, so only the carboxylic acid fizzes.
再加碳酸钠：苯酚和乙醇的酸性都不足以与碳酸盐反应，所以只有羧酸会冒泡。
That fizz is the discriminating test.
这个冒泡就是关键的鉴别反应。
Now a real exam question, worth four marks.
现在来一道真题，四分。
Put four compounds in order of acidity: benzoic acid, phenylmethanol, four-methylphenol and water, and explain your answer.
把四种化合物按酸性排序：苯甲酸、苯甲醇、4-甲基苯酚和水， 并解释你的答案。
Pause the video and try it.
先暂停，自己试一试。
Ready?
好了吗？
Rank the anions, not the acids.
要排的是负离子，不是酸本身。
Benzoic acid comes first: its anion spreads the charge over two oxygen atoms.
苯甲酸排第一，因为它的负离子把电荷分散到两个氧原子上。
Next comes four-methylphenol, spreading its charge into a ring.
接下来是 4-甲基苯酚，它把电荷分散进环里。
Then water.
然后是水。
And phenylmethanol is the least acidic — its hydroxyl group sits on a side chain.
而苯甲醇酸性最弱，因为它的羟基长在侧链上。
Now collect the marks the examiner's way.
现在按考官的方式把分数一分一分拿到手。
One mark is the order itself.
第一分是排序本身。
One more is the link: the further the negative charge is spread, the more stable the anion, and the more easily the hydrogen is lost. The last two marks are two reasons, from any two compounds.
再一分是那条联系：负电荷分散得越开，负离子越稳定，氢也就越容易失去。
Charge delocalised over two oxygen atoms in benzoic acid. An oxygen lone pair delocalised into the ring in the phenol. An alkyl group pushing electron density on in phenylmethanol.
最后两分是两条理由，从这些化合物中任选两个来写： 苯甲酸中电荷离域到两个氧原子上；酚中氧的孤对电子离域进入环中； 苯甲醇中烷基把电子密度推向氧。
Every sentence is about the anion, never about the acid.
每一句话讲的都是负离子，而不是酸本身。
That same lone pair does a second job.
同样是那对孤对电子，还做了第二件事。
It does not only stabilise the anion; it also feeds electron density into the ring of phenol itself.
它不仅稳定了负离子， 还把电子密度送进苯酚自己的环里。
The ring becomes more electron rich, so it attracts electrophiles far more strongly.
环变得更富电子，因此更强烈地吸引亲电试剂。
Benzene is sluggish; phenol is eager.
苯很迟钝，苯酚却很积极。
That is why every ring reaction of phenol runs under much milder conditions: dilute reagents, room temperature, and no catalyst.
这就是为什么苯酚的所有环上反应都在温和得多的条件下进行： 稀试剂、室温，而且不需要催化剂。
Watch what that means in a test tube.
看看这在试管里意味着什么。
Bromination: add bromine water to phenol at room temperature, without a catalyst, and the orange colour vanishes while a white precipitate drops out.
溴化：室温下把溴水加到苯酚中，不用任何催化剂， 橙色立刻褪去，同时析出白色沉淀。
The product is tribromophenol: three bromine atoms have replaced three hydrogens, at positions two, four and six.
产物是三溴苯酚： 三个溴原子取代了三个氢，分别在二号、四号和六号位。
Benzene cannot do this.
苯做不到这一点。
Benzene needs pure bromine and a halogen carrier, and even then only one bromine goes on.
苯需要纯溴和卤素载体催化剂，而且即使这样也只上一个溴。
Nitration tells the same story.
硝化讲的是同一个故事。
Phenol needs only dilute nitric acid at room temperature.
苯酚只需要室温下的稀硝酸。
Benzene needs concentrated nitric acid mixed with concentrated sulfuric acid, warmed to about fifty degrees Celsius.
而苯需要浓硝酸与浓硫酸混合，并加热到约五十摄氏度。
And phenol gives two products, not one: two-nitrophenol and four-nitrophenol, because the hydroxyl group directs the new group to positions two, four and six.
而且苯酚给出两种产物，而不是一种：2-硝基苯酚和 4-硝基苯酚的混合物， 因为羟基把新基团导向二号、四号和六号位， 而六号位不过是从另一边看的二号位罢了。
One last reaction, and the diazonium salt comes back.
最后一个反应，重氮盐又回来了。
First dissolve phenol in sodium hydroxide, because the phenoxide ion is richer in electrons still.
先把苯酚溶在氢氧化钠里， 因为苯氧负离子的电子更加丰富。
Keep it cold, add a diazonium salt, and the two rings join through a nitrogen-to-nitrogen link.
保持低温，加入重氮盐， 两个环就通过一个氮氮连接连在一起。
The product is an azo compound: a strongly coloured dye, because the delocalised system now runs across both rings.
产物是偶氮化合物：一种颜色很深的染料， 之所以有颜色，是因为离域体系现在贯穿了两个环。
And naphthol behaves in exactly the same way.
萘酚的表现也完全一样。
Three marks students throw away.
三个学生常丢的分。
First, no fizz with a carbonate: phenol reacts with sodium hydroxide but not with sodium carbonate, and that one fact separates it from a carboxylic acid.
第一，与碳酸盐不冒泡：苯酚与氢氧化钠反应， 但不与碳酸钠反应，仅凭这一点就能把它和羧酸区分开。
Second, explain with the anion, never with the acid — the charge is delocalised into the ring, so the phenoxide is stabilised.
第二，用负离子来解释，绝不要用酸本身来解释——要说电荷离域进入环中， 因此苯氧负离子被稳定。
Third, write the conditions in full: bromine water at room temperature, no catalyst, a white precipitate; dilute nitric acid at room temperature.
第三，条件要写全：室温下的溴水，不用催化剂， 生成带三个溴的白色沉淀；以及室温下的稀硝酸。
Get those right, and this topic is yours.
把这三点做对，这个专题就是你的了。